Fabry-Perot etalons have been used for many years to select and stabilize the wavelength of tunable diode lasers for dense wavelength division multiplexed (DWDM) systems. In order to match the transmission channels of an etalon with the International Telecommunication Union grid, precise measurement of the free spectral range (FSR) of the etalon is critical. Most reported works are based on the mapping out of the transmission spectrum as the injected laser wavelength is tuned as described in H. Jager, M. Musso, C. Neureiter, and L. Windholz, “Optical measurement of the free spectral range and spacing of plane and confocal Fabry-Perot interferometers,” Optical Engineering, 29, 1, pp 42-46, January (1990); P. D. Kinght, A. Cruz-Cabrera, and B. C. Bergner, “High-resolution measurement of the free spectral range of an etalon,” Proceedings of SPIE, 4772, pp 114-117, (2002); and R. Williamson, and C. Terpstra, “Precise free spectral range measurement of telecom etalon,” Proceedings of SPIE, 5180, pp 274-282, (2003).
These prior art techniques are quite simple and fairly precise allowing up to 4 part per million of error for a 100 GHz free spectral range etalon. However, the precision is fundamentally limited by the resolution of the optical spectrum analyzer or tunable laser used, making it very difficult to apply to etalons with a FSR smaller than 10 GHz. The Pound-Drever-Hall (PDH) technique has been well known to stabilize the laser wavelength using an etalon as a frequency reference. The present invention uses a simple modification of PDH to measure the FSR of etalons with precision easily exceeding one part of 104 regardless of the size of FSR.